Magnetic flaw detector system for reciprocating pairs of leakage field detectors with means for adjusting the spacing between each pair of detectors

ABSTRACT

A non-destructive testing system for detecting longitudinal and transverse defects, simultaneously, in metallic tubes or pipes in which a plurality of essentially punctiform magnetic sensing elements are disposed along a generally straight line adjacent the workpiece surface. A mechanical oscillator reciprocates the sensing elements along a line parallel to that of the sensing elements disposition. Adjacent elements are electrically connected in opposition to form differential probe pairs, the line connecting the adjacent probes to form the differential pairs being inclined at 45 degrees to the line of reciprocation. In addition to providing for the simultaneous detection of longitudinal and transverse defects, the system disclosed can resolve the signals from ID and OD defects to a degree heretofore unobtainable.

BACKGROUND OF THE INVENTION

The present invention relates to a system for non-destructive testing ofmetallic workpieces, and, more particularly, for the testing ofelongated cylindrical members such as tubes or pipes, to detect bothlongitudinal and transverse defects.

Procedures are known in the prior art for the magnetic non-destructivetesting of metallic workpieces. These systems utilize one of two methodsof magnetic testing. The first is the so-called eddy current method, inwhich a high-frequency alternating magnetic field is used to produceeddy currents in the workpiece surface layer and the characteristics ofthe eddy currents are related to the physical condition of theworkpiece. A sensing element located near the workpiece produces anelectric signal responsive to the fields reradiated by these eddycurrents.

The second approach is the so-called stray field or leakage flux method,whereby a magnetic field is generated within the workpiece and the fieldin the adjacent air is monitored. Defects which cut across the lines offlux of the magnetic field will generate so-called stray fields adjacentthe surface of the workpiece, and magnetic sensing elements detectingthe stray fields produce an electric signal indicating the presence ofthe defects.

The eddy current method may be used for the testing of any metallicworkpiece; however, since the eddy currents only penetrate a very shortdistance below the surface of the workpiece, it is not useful fortesting the entire thickness of a relatively thick workpiece. The strayfield method must be used on ferromagnetic workpieces; however, it willdetect the presence of a defect located anywhere throughout the entirethickness of the workpiece.

Non-destructive testing systems using both of these methods are alsoknown in the prior art. However, there are many shortcomings associatedwith the known systems which are overcome by the present invention, oneof the more important of these shortcomings being the inability todetect both transversely and longitudinally extending defects. Secondly,those parts of the systems relating to the stray field method frequentlyutilize elongated sensing coils positioned adjacent the surface of theworkpiece, which produce signals that are not proportional to the depthof a defect, whereas standards of the testing art usually require therejection of a workpiece having a defect with a depth which exceedscertain limits. Moreover, the magnitude of a signal produced by such asearch coil depends not only on the size of the defect, but also on therelative speed at which the coil moves over the surface and thedirection of the defect. Still further, when testing elongatedcylindrical workpieces such as pipes, a separate coil must be providedfor each pipe diameter, since the curvature of the coils must match thecurvations of the pipes exactly. Also, known stray field systems havebeen unable to satisfactorily resolve ID and OD defect signals.

OBJECTS AND SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide asystem for the non-destructive testing of metallic workpieces such aspipes and tubes, which can simultaneously detect longitudinal andtransverse defects.

A further object is to provide a tube testing system wherein ID and ODdefect signals may be easily distinguished.

A still further object of the invention is to provide an improved defecttesting system utilizing both leakage flux and eddy current procedures.

Another object of the invention is to provide a system as in the aboveobjects in which punctiform sensing elements scan the workpiecereciprocatingly.

The invention may be effectively used for the testing of a workpiece ofany geometry; however, the system disclosed herein is particularlyadapted for the testing of elongated cylindrical members such as tubesand pipes. In accordance with the practice of this invention, a numberof practically punctiform test elements, i.e., elements which are smallcompared to the defect size, are disposed along a generating line of thetube or pipe. A mechanical movement reciprocator or oscillator connectswith each sensing element to oscillate it in a direction essentiallyparallel to the longitudinal axis of the tube. Relative helical motionis established between the tube and set of sensing elements whereby thetube is scanned by the elements. Adjacent pairs of sensing elements havetheir outputs connected in series opposition so as to compare adjacentpoints on the tube's surface and to produce a composite signalcorresponding to the difference in the magnetic properties of suchadjacent regions. In this manner a characteristic defect signal isgenerated which allows ID and OD defects to be easily distinguished whena stray field or leakage flux system is employed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of apparatus according to the presentinvention for eddy current testing.

FIG. 2 is an enlarged schematic view of a tube under test showingdetailed sensing by a pair of adjacent sensing elements.

FIG. 3 is an elevational view of apparatus according to the inventionfor leakage flux testing.

FIG. 4 shows a detailed schematic view of the tube of FIG. 3, showingthe arrangement of the various magnetic field vectors.

FIG. 5 is a cross-section taken along the line 5--5 of FIG. 3.

FIG. 6 is a plan view of one type of oscillator or reciprocator beam foruse in the present invention.

FIG. 7 is a graphical representation of the radial component of themagnetic stray field produced by a defect.

FIGS. 8a and 8b are graphical representations of the radial component ofthe magnetic stray field of a defect and the corresponding signalvoltage as a differential probe pair scans over the defect with thedefect location at or near the inner surface.

FIGS. 9a and 9b are similar to FIGS. 8a and 8b except that the defect isat or near the outer surface.

FIG. 10 depicts one form of magnetic sensor for use with this invention.

FIG. 11 illustrates a circuit schematic for identifying the type ofdefects detected by the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a system for the inspection of metallic tubes using theeddy current method according to the present invention which isespecially useful for the detection of surface defects. Such surfacedefects are encountered, for instance, in oil well drill strings whichare exposed to bending movements during drilling which produces externalfatigue cracks. Accordingly, the drill strings are frequently checkedfor fatigue defects after each operation.

The tube 1 to be tested can be rotated by a device (not illustrated)about the axis of the tube in the direction of the arrow 2. The devicefor the rotation of the tube may, in the case of oil well pipes forexample, consist of two double rollers, each of which is mounted in thehead of a hydraulic or pneumatic lifting device, the lifting devicemoving the tube to be tested from a pile of such tubes into the testposition. At the same time the tube is laid on the double rollers whichform the heads of the respective lifting devices. The double rollers aredriven by means of an attached motor and servo to rotate the tubesduring testing.

Guide wheels 13 are freely mounted in the carriage 3 and contact thetube 1 during the testing operation with the wheel axes disposed at anangle to the longitudinal axis of the tube. The carriage 3 is rotatablymounted in a fork 5 as at 4, which fork 5 is positively connected to thearm 6 rotatably suspended in the bearing 7 of a frame 8 and is therebyfree to rotate through a limited angle in these bearings. The frame 8 isfixedly secured to a guide 9 which slides on the shafts 10 and 11longitudinally with respect to the tube 1. A drive (not illustrated)positions the guide 9 in the direction of arrow 12.

A reciprocator or oscillator beam 17 is pivotally suspended at its endsby a first lever 15 and a bent lever 16 which are in turn pivotallymounted at the points 17 and 18, respectively, on carriage 3. The drivearm 19 of the bent lever 16 is eccentrically related to the disc 20which is rotated by a motor (not shown) in the direction of the arrow21. Eight eddy current probes 22 and 29 are carried by the beam 14,arranged in two lines of four probes each, with the probes in one linebeing offset axially of the tube relative to the probes in the otherline. The amount of the offset is the same as the distance between thetwo lines of probes so that four pairs of probes are formed which areadjacent each other on lines forming a 45 degree angle with thelongitudinal axis of the tube.

The adjacent pairs of probes, i.e., 22 and 23, 24 and 25, 26 and 27, and28 and 29, have their outputs connected in series opposition to eachother, whereby four so-called differential probe pairs are formed.

In operation, the tube 1 rotates in the direction of the arrow 2 and theguide 9 moves over the shafts 10 and 11 in the direction of the arrow 12and, thus, the carriage 3 moves axially along the tube 1 in the samedirection. Accordingly, the oscillator beam 14, with probes 22-29 scansan essentially helical band on the surface of tube 1 while oscillatinglongitudinally in the direction of the arrow 30.

The oscillator may have, for example, a frequency of oscillation of 50cycles per second and the tubes may rotate with a circumferential speedof about 150 centimeters per second. Each probe will then trace azig-zag line on the surface of the tube, and the circumferentialdistance from peak to peak of this line will be approximately 3centimeters. The distance between the adjacent probe in the direction ofthe oscillator movement is in the present embodiment about 60 percent ofthe maximum amplitude of the oscillator. In this way, the surface of thetube 1 is covered with a net of eight closely spaced and slightlyoverlapping zig-zag lines which results in a thorough tracing of thetube surface, assuring detection of all significant defects on thesurface. Moreover, the tube is scanned in a helical band having a widthequal to the width of the entire set of probes plus the oscillatoramplitude. The rate of feed of guide 9 is chosen so that the carriage 3moves a distance corresponding to just less than the width of thehelical band during pne revolution of the tube.

If the object of the inspection were the detection of defects extendingtransversely of the tube length only, the differential probe pairs couldbe arranged in one line along a generating line of the tube 1. However,the arrangement of the probe pairs in lines which are inclined at 45° toa generating line of the tube permits a well-defined indication ofdefects extending in a number of possible angular directions as relatedto the generating line of the tube. FIG. 2 shows a transverse defect 35and a longitudinal defeat 36 alongside the differential probe pairconsisting of the single probes 22 and 23. Because the probes areoscillating in the direction of the arrow 30, first probe 23 and thenprobe 22 will cross the defect 35, resulting in a well definedindication of the transverse defect. Because the tube 1 is rotatingabout its axis in the direction 2, first probe 23 and then probe 22 willtraverse the defect 36. Therefore, a well defined indication of alongitudinal defect will be provided as well.

The invention described to this point may also be employed with themagnetic leakage flux techniques to detect defects situated other thanat the outer surface of the tube. In principle, an arrangement similarto that previously described could be used except that leakage fluxdifferential probe pairs would be substituted for the eddy currentprobes 22-29. Additionally, provision must be made for a device to applya magnetic field to the tube which can be fairly large, depending on thesize of the tube being tested. Therefore, it is generally more expedientto provide an installation with stationary magnetizing means and fixedlymounted scanning means through which the tube is driven.

Such an installation is illustrated in FIGS. 3 and 5, in which a liveroller bed 40 is traversed by a tube 41 to be tested in the direction ofthe arrow 42. The tube 41 rests on rollers 43 driven by a motor (notshown) in the direction of the arrows 44. The axes of the rollers areslightly inclined relative to the axis of the tube, so that the tubeundergoes not only rotation, but an axial movement as well, i.e.,experiences a resultant forward helical motion in the direction of thearrow 42. The pitch of the helix depends on the inclination of therollers 43 and is preferably adjustable.

Two magnetizing coils 45 and 46 coaxially mount the tube 41 and producea magnetic field B₁ in the tube between the coils, extending along thedirection of the axis of the tube 41 as shown in FIG. 4. A magnetizingyoke 48, shown in cross-section in FIG. 5, provides a transversemagnetic field. The yoke includes a core 51, two extension pole pieces52 and 53 and exciter coils 54 and 55. When a direct current is passedthrough the coils 54 and 55, the yoke produces a transverse magneticfield between the pole pieces and in the wall of the tube being tested.In FIG. 4, this magnetic field is represented by the vector B_(g). Inthe region of the workpiece halfway between the pole pieces 52 and 53and halfway between the magnetizing coils 45 and 46, the two magneticfields B₁ and B_(g) will produce the resultant field B_(res).Preferably, the vectors B₁ and B_(g) are equal in magnitude so that theresultant magnetic field is inclined at an angle of 45° to the tubeaxis.

A mechanical reciprocator or oscillator arrangement 60, having anoscillator beam 61, is provided which operates substantially identicallyto that shown in FIG. 1. The oscillator is mounted above the tube 41halfway between the two magnetizing coils 45 and 46 and midway betweenthe pole pieces 52 and 53. For reasons of simplicity, the oscillatordrive has not been illustrated and may be the same as for the firstdescribed embodiment and produces a reciprocating movement in thedirection of the arrow 62. Four differential probe pairs sensitive toleakage flux and with the line between the probes of each pair inclinedat an angle of 45° to a generating line of the tube 41 are mounted onthe oscillator beam 61. The probes are so oriented that only the radialcomponent of the leakage flux, i.e., the field component normal to thesurface of the tube, will be sensed. The advantages of this arrangementwill be more fully described below.

In operation of the system, the tube 41 is driven by the rollers 43 andmoves helically through the coils 45 and 46 and the yoke 48 and belowthe oscillator means 60. Owing to the vectorial addition of the twofields produced by the magnetizing device, a magnetic field B_(res) iscreated at the point in the workpiece under the oscillator 60, thedirection of said field being inclined at 45° to the longitudinal axisof the tube, which field orientation causes transverse and longitudinaldefects to produce equal stray fields. Accordingly, transverse andlongitudinal defects can be detected with equal sensitivity.

While the tube is moved forward helically, the oscillator beam carryingthe field sensitive elements 64-71 traces a ribbon or band on thesurface of the tube as previously described, the width of the ribboncorresponding to the width of the set of probes plus the oscillatoramplitude.

FIG. 6 shows a further embodiment of the oscillator beam 61 in planview, which is particularly advantageous for use in a leakage fluxtesting system. The characteristic feature of this embodiment is thatthe distance between the single probes in a differential pair can beadjusted while maintaining the angle of the line between them at 45° tothe tube axis. In addition, this angle may be changed to 135° to thetube axis, and the distance between the two single probes adjusted atthis angle, as well. To this end, the probes 64, 66, 68 and 70 arefixedly mounted to the probe beam 61. The probes 65, 67, 69 and 71 aremounted on an adjustable comb 72, which is positioned in a recess 73 ofthe beam 61 disposing these probes all at the same distance from thesurface of the tube being tested. Two continuous guide grooves 74, oneat each end of the comb 72, are formed in the comb. One part of eachgroove is inclined at an angle of 45 degrees to the tube axis while asecond portion is inclined at an angle of 135° to the same axis, withthe inclined parts connected by a straight portion running parallel tothe tube axis. A knurled cap screw 75 passes through each groove 74 andis threaded into the oscillator beam 61 to guide the comb 72 via theguide groove 74 and to fix the position of the comb after the desireddistance and angle have been selected.

An explanation of the particular advantages of this type of oscillatorbar requires a brief explanation of the character of the magnetic strayfield produced by a defect. As previously explained, the magneticleakage flux probes are aligned so that only the radial component of astray field is sensed. With reference now to FIG. 7, in a section of thetube wall 81 there is a defect 82 at a distance T below the outersurface. The path of a probe is indicated by the dashed line 83 at adistance h above the surface. A graph of the magnitude of the radialcomponent of the stray field produced by the defect is superimposed onthis line, with the line 83 representing the zero level, and shows thatit varies from the zero level 83 to a positive maximum 83a, back to zeroat 83b, to a negative minimum value 83c and back to the zero level 83.The maximum 83a and the minimum 83c are spatially located at a distanced from each other.

It can be shown that the distance d between the maximum and minimumequals the sum of the depth T and the distance h, i.e., d=h+T.Therefore, in the case of an ID defect the distance d is large, whereasin the case of an OD defect the distance d is relatively small. Thisfact may be used to differentiate between the signals from ID to ODdefects. Also, the distance between the single probes in a differentialprobe pair may be adjusted to correspond to the distance d between themaximum and minimum of a defect as a certain depth for a purpose thatwill be more fully explained below.

FIG. 8a depicts a plot 91 of the radial component of the leakage fluxcaused by a defect substantially below the tube surface. The two probes92 and 93 of a differential pair are adjustably located at a distance dfrom each other, which is equal to the distance between the maximum andminimum of the radial component of the leakage flux produced by thedefect. If the defect is on the inside surface of the tube, thisdistance d is equal to the thickness of the wall plus the height of theprobes above the tube surface. The point 94 represents the midpoint ofthe line joining the two probes. The probe 92 is the positively actingprobe of the differential pair, i.e., it will generate a positive signalwhen the radial component of the leakage flux is positive, whereas theprobe 93 is the negatively acting probe of the pair and it will generatea negative signal when the radial component of the leakage flux ispositive. The two signals, it will be remembered, are added together inopposition, so that if each of the probes of a differential pair sensesan equal field, the net output from the pair will be zero.

FIG. 8b represents a plot 95 of the output voltage U_(d) from thedifferential pair 92, 93. The abscissa on the graph represents theposition of the probe pair centerpoint 94 on the surface, while theordinate represents the output voltage. It will be seen that the outputvoltage as the probe pair scans over the defect varies from zero to anegative value in the range 96, zero, a maximum 95, back to zero in therange 97, again from zero to a minimum 98 and back to zero. The maximum95 corresponds to the probe position shown in the upper part of FIG. 8.

In analogous fashion, FIGS. 9a and 9b illustrate a plot 101 of theradial component of the leakage flux and a plot 103 of the outputvoltage caused by a defect near the surface. FIG. 9a shows the magnitudeof the radial component of the stray field while FIG. 9b shows theoutput voltage from the differential probe pair 92, 93. The probespacing is the same as in FIG. 8, but the distance between the maximumand minimum of the radial component d, is considerably smaller, whichresults in the radically different curve of the form shown in FIG. 9b.This curve 103 shows two maxima 102 in the range 97', whereas the curvein FIG. 8b has but one. In addition, the signal from an ID defect as inFIG. 8b has a maximum 95 which is approximately twice as high as thesignal generated in a single probe 92 or 93. This is due to the factthat when the probe pair center 94 is directly over the defect, thepositively acting probe 92 is located at the maximum of the radialcomponent of the stray field and the negatively acting probe 93 islocated at the minimum of the radial component of the stray field. Thenegatively acting probe 93, on sensing a negative field, produces apositive signal when it is added to the positive signal from the probe92 to give a higher maximum at 95. In contrast, when the probe pair arecentered over a surface defect, the probes are displaced from themaximum positions for a single probe and therefore the recorded maxima102 in FIG. 9b are only slightly greater than the signal that would beobtained from a single probe. The two signals are also different in thatthe curve 103 shown in FIG. 9b has a much greater percentage of highfrequency components than the curve 95 of FIG. 8b.

The two effects just discussed allow a much finer distinction of ID fromOD defects, and a much more sensitive detection of ID defects than hasheretofore been possible. In the first place, the preferred mutualspacing of the probes of the differential pair enhances the signalarising from an ID defect as described above, but does not increase thesignal from an OD defect. This is exactly what is desired, for thesignal from an ID defect has in past systems always been much weakerthan an OD defect signal. To overcome this disparity in signalmagnitudes, previous known leakage flux systems have resorted to complexamplification systems to enhance the signals originating from IDdefects, whereas the present invention obtains improved results withconsiderable simplification of the electronics and consequent reductionin cost.

A further advantage results from the fact that the described probespacing produces a large difference in the frequency spectra of ID andOD defects. It has been known to separate ID and OD signals on the basisof their different frequency spectra, but the frequency difference inprior systems has been rather small. For example, certain previoussystems have relied solely on the fact that the leakage flux from an ODdefect presents a steeper gradient than the flux from an ID defect, andwhen such a flux is scanned by a moving probe, the frequency of an ODdefect signal is slightly higher than that from an ID defect. However,the differential probe pair arrangement of the present inventionproduces an OD signal having a relatively high frequency component andwhich, in fact, is approximately twice the frequency of an ID defectsignal. It can be seen by comparing the output signals in FIGS. 8b and9b, that the ID defect signal passes through approximately 11/2 cycleswhile the OD defect signal passes through 21/2 cycles in the same timeperiod.

Referring again to FIG. 6, the adjustable comb 72 allows the distancebetween the individual probes in the differential pairs to be adjustedto correspond to the distance between the maximum and minimum of theradial component of a stray field produced by an ID defect. And, asalready noted, this is equal to the wall thickness plug the distance ofthe probes from the surface. This alows maximum enhancement of an IDdefect signal and maximum separation of the frequency spectra of ID andOD defect signals. Also, with the probe pairs inclined at 45° to thetube axis as previously described, they will react to both longitudinaltransverse cracks or defects.

Still further, the probe pairs will respond to spiral (inclined) defectsas long as they are perpendicular to the direction of the magnetic fluxin the workpiece and to the line joining the probes of the differentialpair, i.e., as long as they are oriented as the crack 115 shown in FIG.2. With such a crack, a leakage flux component is created over thedefect and the individual probes 22 and 23 pass over the defect atdifferent times, as is necessary for a good differential indication ofthe defect. Spiral defects which run in the same direction as themagnetic flux and the connecting line between the single probes will bemissed. However, spiral inclined cracks are encountered only rarely, andit is usually the case that when found they will run in a preferredhelicoidal direction dependent on the manufacturing procedure.

The oscillator bar 61 with the adjustable comb 72 allows the system tobe adapted to detect spiral or select inclined defects. As previouslydescribed, the line between the single probes of the differential pairscan be adjusted to 45° or 135° relative to the tube axis by looseningthe cap screws 75 and shifting the comb 72 until the cap screws ride inthe desired inclined portions of the guide groove 74. The cap screws arethen retightened. Of course, the direction of the magnetic flux in thetube wall must be rotated 90°, and this is accomplished simply byreversing the current to either the longitudinal field coils 45 and 46or to the yoke coils 54 and 55.

Although a number of different probe constructions may be used in thepresent invention, best results to date have been obtained by the use ofdevices operating on the so-called Hall effect principle. With referencenow particularly to FIG. 10, a Hall effect device 120 is seen to includea disc-like insulative substrate 121 on which is mounted a relativelythin disc 122, which is a special semiconductor crystal. A first pair ofleads 123 and 124 connected to opposite sides of the disc 122 provideenergizing current from a direct current source, and a second pair ofleads 125 and 126 connected to opposite sides of the disc 122 and at 90°to the connection for 123, 124 serves as a signal output means. When thedevice is provided with energizing current via leads 123, 124, magneticflux (H) directed at 90° to a major surface of the disc 122 causes avoltage signal to occur across leads 125, 126.

Reference is now made to FIG. 11 depicting the circuit schematic for apair of probes 127 and 128 forming a differential pair, with it beingunderstood that other differential probe pairs of the describedapparatus may be connected in the same manner. The probes are providedwith direct current energization from a battery source 129, for example,via leads 130 and 131, the value of which may be selectively adjusted bythe variable resistor 132. The signal lead from the probe 127 and theoppositely poled signal lead of probe 128 are connected throughcapacitors 133 and 134, respectively, to the input of a differentialamplifier 135. The amplifier output is filtered via filters 136 (ID) and137 (OD) which are appropriately designed to pass signals havingfrequency characteristics of ID and OD defect signals, respectively.Triggers 138 and 139 are actuated by the corresponding filtered outputsof the filters 136 and 137 to, in turn, energize either the ID or ODutilization apparatus 140 and 141. The utilization apparatus can takeany number of different forms, such as, for example, devices forspraying a band or spot on the workpiece of a color coded to identifythe type of defect, means for conveying defective tubing along adifferent path from that of tubing with no defects, or the like.

What is claimed is:
 1. Apparatus for detecting defects in the walls ofmetal tubing, comprising:means for generating a magnetic field withinthe body of said tubing whereby a stray field is produced lying radiallyoutwardly of said tubing adjacent defects in said tubing; a plurality ofpaired sets of magnetic field sensing devices arranged in spacedrelation to the outer surface of said tubing, the devices of each pairbeing electrically connected in opposition and positioned relative toeach other and the test pieces such that a straight line connecting thedevices of each pair would be oriented at an acute angle with respect tothe tubing longitudinal axis; means for adjusting the spacing betweenpaired sets of devices; means for producing relative helicoidal movementbetween the tubing and the sensing devices; and means for reciprocatingthe field sensing devices generally longitudinally of the tubing duringrelative movement.
 2. Apparatus as in claim 1, in which the devices ofeach pair are separated from one another at a distance substantiallyequal to the sum of the tubing wall thickness and the spacing of thedevices from the outer tubing wall.
 3. Apparatus as in claim 1, in whichthe pairs of devices are arranged in paired sets extending consecutivelyalong a line parallel to the tube axis and with the line between devicesof each pair being oriented angularly with respect to the tubing axis ata common angle of substantially 45°; and the means for reciprocatingconnected to move all of the sensing devices as a single unit. 4.Apparatus as in claim 1, in which the devices of each paired set arearranged in mutually spaced relation, and the field sensing devices arereciprocated longitudinally of the tubing an amount bearing asignificant ratio to the spacing of the paired devices from each other.5. Apparatus for the nondestructive testing of metal tubes to locatelongitudinal and transverse defects, comprising:means for moving a tubeto be tested along a helicoidal path; a carriage mounted upon the outersurface of said tube; reciprocating means mounted on said carriage; atleast one pair of mutually spaced magnetic field sensing elementscarried by the reciprocating means and disposed in slightly spacedrelation to the outer surface of said tube, means for adjusting andspacing between said elements, said elements being arranged in a lineextending generally at 45° to the tube axis, and said elements beingconnected in electrical opposition to form a differential pair wherebythe signal obtained from the pair of elements corresponds to thedifference in the defect characteristics of the two regions being sensedby the elements; and electric circuit means connected to receive signalsfrom said sensing elements and including a first channel responsive tothe frequency characteristic of a tube outer surface defect and a secondchannel responsive to the frequency characteristics of signals caused bydefects other than on the tube outer surface.
 6. Apparatus as in claim5, in which means are provided on said carriage for selectively changingthe arrangement of said elements from 45° to 135°.
 7. Apparatus as inclaim 5, in which the sensing elements are spaced from each other adistance equal substantially to the sum of the tube wall thickness andthe spacing of the elements from the outer surface of the tube. 8.Apparatus for the nondestructive testing of metal tubes to locatelongitudinal and transverse defects, comprising:means for moving a tubeto be tested along a helicoidal path; a carriage mounted upon the outersurface of said tube; reciprocating means mounted on said carriage; atleast one pair of mutually spaced magnetic field sensing elementscarried by the reciprocating means and disposed in slightly spacedrelation to the outer surface of said tube, means for adjusting thespacing between said elements, said elements being arranged in a lineextending generally at 45° to the tube axis, and said elements beingconnected in electrical opposition to form a differential pair wherebythe signal obtained from the pair of elements corresponds to thedifference in the defect characteristics of the two regions being sensedby the elements; and electric circuit means connected to receive signalsfrom said sensing elements.